The present invention relates to a multi-layer substrate used in a millimeter wave band and a microwave band, and more specifically to a multi-layer substrate configured to suppress an unwanted propagation mode.
In the prior art, a composite microwave integrated circuit having a number of semiconductor devices mounted on a multi-layer substrate is known, which was disclosed in Japanese Patent Application Pre-examination Publication No. JP-A-09-167825. In the circuit disclosed in this patent publication, a plurality of semiconductor devices are accommodated in a cavity formed in a dielectric substrate, so as to avoid influence of an external electromagnetic field and to ensure an airtight condition. Each of the semiconductor devices are connected to microstrip lines.
In addition, T. Hirose et al, xe2x80x9cA FLIP-CHIP MMIC DESIGN WITH CPW TECHNOLOGY IN THE W-BANDxe2x80x9d, IEEE MTT-S, International Microwave Symposium Digest, pp.525-528, 1998 (the content of which is incorporated by reference in its entirety into this application) discloses a high frequency circuit including a coplanar waveguide formed on a single-layer substrate as an interconnection line. In a coplanar waveguide line, a signal line and a ground line are formed in the same plane, and the coplanar waveguide line is suitable to a flip chip bonding which can make connection with a low inductance and good repeatability.
When the coplanar waveguide line is used, however, a leakage of a signal wave in an unwanted propagation mode within the substrate has become a problem. This problem of leakage is described in detail in H. Shigesawa et al, xe2x80x9cCONDUCTOR-BACKED SLOT LINE AND COPLANAR WAVEGUIDE: DANGERS AND FULL-WAVE ANALYSESxe2x80x9d, IEEE MTT-S, International Microwave Symposium Digest, pp.199-202, 1988 (the content of which is incorporated by reference in its entirety into this application). The leakage mode includes a surface wave mode occurring when a ground conductor is formed on only one surface of a dielectric plate (as the ground conductor constituting the coplanar waveguide line) and a parallel-plate mode occurring when a ground conductor is formed on each of opposite surfaces of the dielectric plate (as the ground conductor constituting the coplanar waveguide line, and a ground conductor formed on a back surface of the dielectric plate). The leakage in these modes increases a transmission loss in the coplanar waveguide line, and causes interference between adjacent lines in the substrate, between adjacent devices in the substrate, and/or between adjacent line and device in the substrate.
Under the above mentioned circumstances, various approaches for suppressing the unwanted modes in the case that the coplanar waveguide line is formed in a single-layer substrate or in a multi-layer substrate, have been proposed.
A method for suppressing the surface wave mode is described in, for example, Misao HANEISHI, xe2x80x9cModern Planar Antennaxe2x80x9d, Kabushiki Kaisha Sogou Gijutsu Center, Page 63. In this literature, it was designed so that the following equation (1) holds:
t less than c/{4fxc2x7(∈rxe2x88x921)e,fra 1/2}xe2x80x83xe2x80x83(1)
where t is a thickness of a dielectric substrate
c is velocity of light
f is an operating frequency
∈r is a dielectric constant of a dielectric substrate material
Alternatively, a method for suppressing the parallel-plate mode is described in, for example, N. K. Das, xe2x80x9cMethods of Suppression or 10 Avoidance of Parallel-Plate Power Leakage from Conductor-Backed Transmission Linesxe2x80x9d, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 44, NO. 2, pp. 169-181, FEBRUARY 1996 (the content of which is incorporated by reference in its entirety into this application). Specifically, there are a method for interconnecting, by means of a via contact, between a ground conductor of a coplanar waveguide line and a back surface ground electrode which are formed on opposite surfaces of a dielectric substrate, respectively, and another method for forming a coplanar waveguide line on a double-layer substrate consisting of two layers having different dielectric constants.
However, a problem occurring when the coplanar waveguide line is formed on an internal layer within a multi-layer substrate, cannot be dissolved with only the above mentioned methods.
Here, referring to FIG. 18A, there is shown a diagrammatic plan view of a prior art multi-layer substrate in which the coplanar waveguide line is formed. FIG. 18B is a sectional view taken along the line Hxe2x80x94H in FIG. 18A. The prior art multi-layer substrate is generally designated with the reference number 101, and includes a signal line 107 is selectively formed on a surface of a first dielectric layer 103, and a pair of ground conductor layers 108 formed on opposite sides of the signal line 107, separately from the signal line 107, the signal line 107 and the ground conductor layers 108 being formed from the same conductor layer 104. Thus, a coplanar waveguide line 102 is formed. Furthermore, a second dielectric layer 105 is formed on the coplanar waveguide line 102 to cover the signal line 107 and the ground conductor layers 108.
In order to suppress the surface wave mode in the multi-layer substrate of this structure, the thickness of each of the first and second dielectric layers 103 and 105 must be smaller than the value of xe2x80x9ctxe2x80x9d obtained from the equation (1). Therefore, in order to use a high signal frequency, it is necessary to make the thickness of the dielectric layers as thin as possible. Accordingly, a problem occurs in which the number of layers cannot be increased because of the limited thickness of the dielectric layer, even if it is desired to increase the number of layers in the multi-layer substrate. On the other hand, since the thickness of the overall multi-layer substrate becomes thin, it becomes short of a mechanical strength.
Furthermore, various problems have been encountered in a prior art multi-layer high frequency circuit substrate intended to suppress the parallel-plate mode. Referring to FIG. 19A, there is shown a diagrammatic plan view of a prior art multi-layer substrate intended to suppress the parallel-plate mode. FIG. 19B is a sectional view taken along the line Ixe2x80x94I in FIG. 19A.
The prior art multi-layer substrate intended to suppress the parallel-plate mode, is designated with the reference number 111, and includes a signal line 117 is selectively formed on a surface of a first dielectric layer 113, and a pair of ground conductor layers 118 formed on opposite sides of the signal line 117, separately from the signal line 117, the signal line 117 and the ground conductor layers 118 being formed from the same conductor layer 114. Thus, a coplanar waveguide line 112 is formed. Furthermore, a second dielectric layer 115 is formed on the coplanar waveguide line 112 to cover the signal line 117 and the ground conductor layers 118. Ground conductor layers 119 are formed on a back surface of the first dielectric layer 113 and a front surface of the second dielectric layer 115, respectively. Furthermore, via contacts 121 are formed to penetrate through each of the first and second dielectric layers 113 and 115, in order to interconnect between the ground conductors 118 and the ground conductor layers 119. Incidentally, the via contacts 121 are located along an extending direction of the coplanar waveguide line at intervals which is at least not longer than a half of a signal wavelength.
With this arrangement, a structure equivalent to a section of a hollow-pipe waveguide is formed by the via contact 121 and the ground conductor layers 119. In this waveguide structure, however, since it is assumed that a medium having a dielectric constant higher than that of air is used, a propagation in an unwanted waveguide mode occurs at a frequency lower than that at which a waveguide mode occurs in the hollow-pipe waveguide. Considering that for example an alumina substrate having the dielectric constant of 10 is used, an interval of the via contacts is 1.3 mm, and a spacing between the ground conductor layers is 0.65 mm, a signal having a frequency of not less than 40 GHz propagates in the waveguide mode. Accordingly, in order to realize a desired signal transmission in the coplanar waveguide line in a high frequency region, it is necessary to lower the dielectric constant or to reduce the interval of the via contacts and the spacing between the ground conductor layers.
In adjustment of these factors, however, restriction is involved in a performance of a required substrate material, a forming process, a fabricating cost, a yield of production, etc., with the result that attainment is not easy in some high frequency region.
Accordingly, it is an object of the present invention to provide a multi-layer substrate having a coplanar waveguide line, which has overcome the above mentioned problem of the conventional prior art.
Another object of the present invention is to provide a multi-layer substrate having a coplanar waveguide line, which is configured to suppress an unwanted propagation mode in a high frequency band, and which can be easily fabricated and has a satisfactory mechanical strength.
The above and other objects of the present invention are achieved in accordance with the present invention by a multi-layer substrate comprising a first dielectric layer, a coplanar waveguide line formed on a first surface of the first dielectric layer, and a second dielectric layer formed on the first dielectric layer and having an opening positioned at least on a signal conductor of the coplanar waveguide line.
According to another aspect of the present invention, there is provided a multi-layer substrate comprising a first dielectric layer, a coplanar waveguide line formed on a first surface of the first dielectric layer, and a second dielectric layer formed on the first dielectric layer and having a groove positioned in a region at least in conformity with a signal conductor of the coplanar waveguide line.
With the above mentioned arrangement, since the second dielectric layer formed on the first dielectric layer has the opening or the groove positioned in the region in conformity with the signal conductor of the coplanar waveguide line, an unwanted propagation mode including the surface wave mode is suppressed. On the other hand, since the thickness of the whole is not limited, a satisfactory mechanical strength can be ensured.
In the multi-layer substrate in accordance with the present invention, it is preferred that a thickness of the second dielectric layer between a bottom of the groove and the first dielectric layer is smaller than the value of c/{4fxc2x7(∈2xe2x88x921)xc2xd}, where c is velocity of light, f is a frequency of a signal propagating in the coplanar waveguide line, and ∈2 is a dielectric constant of the second dielectric layer.
In addition, it is also preferred that a thickness of the first dielectric layer is smaller than the value of c/{4fxc2x7(∈1xe2x88x921)xc2xd}, where c is velocity of light, f is a frequency of a signal propagating in the coplanar waveguide line, and ∈1 is a dielectric constant of the first dielectric layer.
Furthermore, a dielectric material having a dielectric constant lower than that of the second dielectric layer, is preferably filled in the groove.
Moreover, a second groove is preferably formed on a second surface of the first dielectric layer, in a region at least in conformity with the signal conductor of the coplanar waveguide line.
In this variation, it is preferred that a thickness of the first dielectric layer between a bottom of the second groove and the signal conductor is smaller than the value of c/{4fxc2x7(∈1xe2x88x921)xc2xd}, where c is velocity of light, f is a frequency of a signal propagating in the coplanar waveguide line, and ∈1 is a dielectric constant of the first dielectric layer.
In this variation, furthermore, a second dielectric material having a dielectric constant lower than that of the first dielectric layer is preferably filled in the second groove.
In another variation, a back ground conductor layer can be formed on a second surface of the first dielectric layer.
In this variation, a via contact can be buried in the first dielectric layer to connect between the back ground conductor layer and a ground conductor of the coplanar waveguide line.
Furthermore, it is preferred that the dielectric constant of the first dielectric layer is smaller than that of the second dielectric layer.
In addition, a conductor layer can be formed on the second dielectric layer.
Moreover, a feed-through can be formed to bridge between a pair of second dielectric layer portions, which are positioned on a pair of ground conductors of the coplanar waveguide line, respectively.
In this case, the conductor layer formed on the second dielectric layer can extend onto the feed-through.
Furthermore, the multi-layer substrate can include a connecting means for connecting the conductor layer and the pair of ground conductors of the coplanar waveguide line.